Fuel cells, which are capable of generating electric power with high efficiency even when they are configured as small-sized devices, have been developed as power generation systems of distributed energy sources. However, there is no general infrastructure to supply a hydrogen gas to be used as a fuel for such a fuel cell to generate electric power. For this reason, a hydrogen generation apparatus is installed together with the fuel cell. The hydrogen generation apparatus uses a raw material gas, such as city gas or propane gas supplied from an existing raw material gas infrastructure, to generate a hydrogen-containing gas through a reforming reaction between the raw material gas and water.
Such a hydrogen generation apparatus often includes: a reformer configured to cause a reforming reaction between a raw material gas and water; a shift converter configured to cause a water gas shift reaction between carbon monoxide and steam; and a CO remover configured to oxidize carbon monoxide by using an oxidant which is mainly air, for example. Each of these reactors uses a respective catalyst suitable for their reaction. For example, a Ru catalyst or a Ni catalyst is used in the reformer; a Cu—Zn catalyst is used in the shift converter; and a Ru catalyst is used in the CO remover. These reactors are used at respective suitable temperatures. Typically, the reformer is used at a temperature of approximately 600° C. to 700° C.; the shift converter is used at a temperature of approximately 350° C. to 200° C.; and the CO remover is used at a temperature of approximately 200° C. to 100° C. Electrode contamination due to CO tends to occur particularly in a solid polymer fuel cell. Therefore, a CO concentration in a hydrogen-containing gas to be supplied to a solid polymer fuel cell needs to be suppressed to several tens of vol ppm. The CO remover reduces the CO concentration by oxidizing CO.
A raw material gas such as city gas contains sulfur compounds. It is necessary to remove the sulfur compounds from the raw material gas in some way since, in particular, the sulfur compounds contaminate a reforming catalyst. In this respect, various proposal have been made including: a method of removing sulfur compounds through ordinary-temperature adsorption (see Patent Literature 1, for example); and a hydrogen generation apparatus configured to perform ordinary-temperature adsorption desulfurization at start-up and switch the desulfurization mode to hydrodesulfurization when ready to generate hydrogen (see Patent Literature 2, for example)
It is well known that when catalysts come into contact with air, oxidation occurs, causing reduced catalyst activity in varying degrees. Therefore, hydrogen generation apparatuses are designed such that when a hydrogen generation apparatus is not operating, the hydrogen generation apparatus is closed by using valves or the like. In such a manner, air is prevented from flowing into the hydrogen generation apparatus. When a hydrogen generation apparatus that is operating is stopped, a pressure drop is caused due to a temperature drop and/or a reaction. There is a proposed hydrogen generation apparatus that solves such a pressure drop. The proposed hydrogen generation apparatus compensates for such a pressure drop by supplying a raw material gas to a reformer (hereinafter, referred to as a pressure compensation operation) (see Patent Literature 3, for example). It is known that a reduction in catalyst activity is also caused by condensation of water. In this respect, there is a proposed hydrogen generation apparatus configured to purge the inside of a reformer by using a raw material gas (hereinafter, referred to as raw material gas purge), thereby preventing catalyst degradation due to dew condensation (see Patent Literature 4, for example).